Investigation of single and tandem flapping hydrofoils

Abstract

Marine vertebrates are well known to generate thrust via the oscillatory motion of bladelike body parts (e.g flippers, tails, fins, etc.). These configurations, often depicted as flapping floils in a two dimensional domain, demonstrate impressive manoeuvrability (Read et al., 2003) without the control imprecision exhibited by contemporary manmade designs (Kato, 1998). In this project, we investigate a bio-inspired approach of underwater propulsion, using single and tandem configurations of periodically flapping hydrofoils. Our main goal is to develop a fundamental understanding of the complex hydrodynamic interactions among shedding vortices that lead to the thrust generation of these systems. To this end we numerically analyse the wake development of the 2D single flapping foil under the influence of varying harmonic and non-harmonic kinematics and profile thickness. We find that the cycle-averaged trajectory T of the foil is sufficient to characterise the drag to thrust wake transition regardless of the chosen periodic motion but only for thicknesses ∼ 10% − 16% of the chord length due to limitations of the two dimensional field assumption. Moreover, we investigate the 2D interaction of deflected wakes in a tandem configuration. Our computational results reveal that a properly tuned back foil is capable of cancelling the wake deflection and mean side force of the front foil, with a subsequent increase in the overall thrust of the system. Finally, we expand the results of a previous study (Muscutt et al., 2017b) by analysing the effect of aspect ratio (AR) on a paleobiomimetic system of tandem foils. In a similar fashion to steady wing configurations, flume tank experiments suggest an improvement in terms of efficiency and thrust coefficient with increasing AR accompanied by a noticeable penalty in manoeuvrability. Yet vortex breaking along the span of highly elongated flippers mitigates these benefits as more energy is diverted towards side force.

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ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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